The broad aim of this project is to isolate and characterize defective deletion mutants of murine sarcoma-leukemia viruses. Cloned infectious viruses have been serially propagated in tissue culture and the extent of expression of the integrated viral DNA as extracellular viral specific RNA is now being examined by molecular hybridization techniques. Full length viral specific circular DNA was isolated from cells infected de novo with virions and cleaved with restriction endonucleases. These DNA fragments were electrophoretically separated gels, transferred and fixed on to nitrocellulose membrane filters and are now being examined for complementarity with radioactive viral RNA and c-DNA prepared with virions isolated at different passage levels. Those virion populations containing defective genome will be isolated and their role on the replication and integration of the DNA of infectious virions will be studied.